Lactoferrin (LF) is a 78,000 Da iron-binding protein which is ubiquitously expressed in exocrine secretions. it is been shown to be bacteriostatic, and is thought to play a role in maintaining the sterility of those secretions in which it appears. it has been shown to have in vitro activity against both gram positive an gram negative bacteria, as well as fungal species. Because its antibacterial activity is seen only when LF is not saturated by iron, it has been presumed that its antimicrobial properties are mediated at least in part by sequestering iron away from potential infectious agents. In addition, however, LF has been shown to interact with a wide range of components of the host response to infection, including effects on natural killer cell activity, on neutrophil number and function, and on monocyte/macrophage activation. The way in which these direct and indirect effects on the host response to infection interact in vivo, and their physiologic importance tot he whole organism remains unclear. We propose to clarify the physiologic function of LF by establishing a model of LF deficiency. This will allow the assessment of the importance of LF in vivo in an intact animal. This should provide insight into the potential contribution of LF as a food or formula additive, as well as revealing other potential pharmacologic applications of the recombinant protein. our specific aims are: 1. To develop a line of LF-"null" mice by targeted disruption of the gene by homologous recombination in embryonic stem cells. Cells containing a hemizygous LF-knockout will be implanted into blastocysts, and the resultant chimeric mice bred to obtain progeny carrying a homozygous LF deletion. 2. To examine the phenotype of homozygous LF-knockout mice for evidence of sequelae of LF deficiency. We will assess the host response of these mice to induced acute bacterial infection in a system which depends on the sequential activation and killing by NK cells, neutrophils, and monocyte/macrophages. 3. To assess the impact of nursing LF-free milk by assessing iron absorption and intestinal flora of offspring of LF-null animals.